NEED dual batteries for fridge?

[latinoguy]

Worth the mod for peace of mind for solo trips

I am adding an extra battery to mine very soon. I figure even without a 12v fridge yet (other mods first) It is worth doing for those solo remote camping outings and trips. I would hate to have to walk 20 miles because I killed the battery on my rig. worse yet, leave my truck unatended in Mexico somewhere because of a dead battery... not that this will be your situation.

 

[BajaTaco]

Just a slight clarification to your calculations, Chris. Were you are measuring current on a running fridge, i.e. reading instantaneous current, or were you actually using a power meter (measuring watts, A-hr, etc.)?

I've measured my Engel to typically draw as much as 2.5 amps while running (it varied, but never saw more than about 2.5A or so). If it runs continuously for one hour, that is 2.5 amp-hours. If it runs for 15 minutes per hour, that's 0.625 amp-hours (i.e. 25% duty cycle). In the summer it generally runs at about 50% duty cycle during the main part of the day and at night it's more like 20%. So I'd guess the real overall duty cycle is about 35~40%. So a battery that can do a 20 hour rate of 55 A-hr would run my fridge with a 40% duty cycle (i.e. an average of 1 A-hr, 0.40 x 2.5 = 1 A-hr) for more like 55 hours, assuming a 1:1 conversion of the 20 hour rate for lower draws.

This model matches well to empirical data (i.e., packing the Engel full of beer and sitting at camp site drinking until my battery reads around 11.5V on a 44 amp-hour Red Top) is about a whole weekend (say around 36 hours). My measurements are very precise for about 6 hours, then get hard to read and completely tail off after about 12. But end of session numbers are once again relatively accurate.

I used a testing device made specifically for capturing the data for the fridge article. I thought about using a stepped-down AC bench power supply, but instead decided on 12V deep-cycle batteries to better simulate the real thing. There were two test circuits, side-by-side, hooked up to two identical (load tested) brand new Deka Group 31 batteries so I could test two fridges at once (which I especially wanted to do with an ARB next to an Engel). Amps, voltage, and temperature were monitored using data-logging software hooked up to a laptop, and sampled at a rate of 0.76hz (once every 1.32 seconds). Additionally, the start-up amperages were recorded by increasing the sample rate to 30hz (30 times per second). These are much higher spikes to start the motor, and occur every time the fridge cycles (turns on) within the one-hour period, so they should also be taken into account (the ARB/Engel fridges spiked at 8.13A). They don't increase the average AH by a whole lot, but it's still worth considering. I know this is a long answer to your question, but I thought I'd elaborate on how I got the numbers. I don't disagree with your numbers though, especially if the fridge is loaded with goods (thermal mass) that have cooled down completely (all the way through the middle) which will make the fridge work much less. There are also lots of variables such as sunlight/shade, humidity, air temps, wire sizes, battery condition, insulated cover (or not), frequency of opening the lid, duration of opening the lid, amount of cooled contents, temp. settings, air circulation, etc.

 

[BajaTaco]

Oh Baja your always ahead of me. I have a hookup with a coast guard buddy and some solar panels pulled off channel markers. He gave me the numbers and I ran them against what I had and it should support the battery for the Engel for a 24 hour period. SHOULD is loosely used and for free I'll try it.

Aaron

Score! So let us know how it goes with the solar. :sunny:

 

[BajaTaco]

This model matches well to empirical data (i.e., packing the Engel full of beer and sitting at camp site drinking until my battery reads around 11.5V on a 44 amp-hour Red Top) is about a whole weekend (say around 36 hours). My measurements are very precise for about 6 hours, then get hard to read and completely tail off after about 12. But end of session numbers are once again relatively accurate.

11.5V is less than 20% state of charge, so you're severely shortening the life of the battery, even more so because it's not a deep-cycle model. You and I both need to get some solar panels.

:suning:

 

[BajaTaco]

Dave, this has been a fun discussion with you It's great that you've taken the time to tinker with your fridge and battery to figure out what's going on so you have a good idea of where you stand with regards to running your fridge on a single battery. I recommend that anyone considering this option do the same, or at least bring along a jump-start pack or travel with someone who can give a jump-start. The Engel/ARB/Norcold 45 fridges are quite the power misers when you make an effort to optimize everything. Yes, the data I collected sampled throughout the entire cycle, so I could see amps while running, duration of cycles, and most importantly, averaging the Amperage per hour for a true AH average which was as little as 1.8AH (cycling) and as much as 2.65AH (continuously running). Check out the article for all of the details on the parameters of the different tests I did. There was a race, a "workhorse" test, a warm-up test, and cycling measurements.

 

[BajaTaco]

My wife, God love her, is no longer shocked when she comes into the basement and sees something like our new $700 MT45 in pieces, connected to owned and borrowed test equipment, baring an uncanny similarity to a patient on the operating table with current inductors and break-out boxes characterizing it's operation.

:yikes: :xxrotflma You should keep going with that, and build a FrankenFridge with full documentation. Freeze steaks on only miliamps

 

[762X39]

(as an aside I HIGHLY discourage people to attempt using an inverter to run a fridge).

I am going to assume you are refering to those cheap modified squarewave inverters that everyone buys because they are "cheap".I run a true sinewave inverter (it cost about 14 times as much as the cheap modified squarewave units) and find it works well in every application while being reasonably efficient. Of course it is powered from a 24vdc source and I believe that if you have a Euro-spec truck you can order your Engel in 24vdc.

 

[Jacket]

:bowdown: :bowdown: :bowdown:

 

[762X39]

OK, fair enough, I threw out a flippant unsupported remark.

It's okay, I'm not trying to piss in anyones cornflakes.As someone with an engineering background, I tend to jump on unsupported remarks (LOL).I checked for harmonics on my inverter and found that other than a bit of 2nd harmonics (very cool when it is a Gibson Les Paul and a Hiwatt Amp) and to a much lesser degree some odd order harmonics (below the "I care threshold") it puts out AC current as good as Ontario Hydro.

 

[BiG BoB]

Is it completely necessary to run dual batts if I plan to run an Engel 45 or similar?

That depends on how long you want to run it for.... from reading your posts it seems like its for overnight stops (with the vehicle being run every day)

Could I get away with 1 high quality deep cycle, such as an Oddysey?

Yes, for an overnight stop with an already cold fridge, provided the battery is in good condition (I like Deka's)

Just trying to sort through the truly needed mods vs. the useful but sort of over the top ones.

Never rely on somebody else being able to give you a jump start, or any other help. On this board there seems to be quite a few members with automatic transmissions, these vehicles are inherantly dangerous and I would never willingly take one out of suburbia. Anyone with an automatic transmission should never travel alone, and should fit a dual battery system.

Sean

 

[762X39]

Taking this thread back on track.....
If you are careful and have a backup plan (ie: a jumpstart box or something) you can run your fridge on a single battery but remember that it is also your starting battery and it is not built to discharge too deeply so you will likely shorten its life. If you are willing to accept this risk and it's associated cost then running a single battery is acceptable. If you tend to park your truck very far from the madding crowds and don't have a backup plan then it may be better that you have a separate house battery or batteries so that you can keep the beer cold without having to hop on your mountain bike and ride 40 km to get your truck boosted.I run two starting batteries and keep the house batteries completely separate with a solar panel and controller to keep them up but I am the exception rather than the rule.My younger brother is even more anal than I (his boat runs 6 batteries as well) but than he trained as an aerospace engineer so he tends to over analyze stuff and run more what ifs than I do.

 

[BajaTaco]

Here is another good thread on this topic. Rob goes into great detail and has an excellent example of how Puekert's law effects the AH rating of the battery.

 

[BajaTaco]

When I plug that into the generalized equation, using n = 1.04, 50 amp capacity, 20-hour rating, etc. I get, for 2 A discharge, a Peukert Capacity of 51.8 A-hr or at 2A, an available discharge rating of 50.45 amp-hours. The time to dead of 25.22 hours, allowing 100% DoD. If I only allow for a 50% DoD, we're talking about 12.5 hours.

That's awesome Dave! :clapsmile

 

[teotwaki]

I should note that my calculated discharge rate is 2 amps continuous. If I make a correction and assume that I turn it down at night, so a rate more like 1.25 A-hr, my life goes up to 41.1 hours at 100% DoD or 26 hours down to 65% DoD. So I think the numbers are very realistic.

So does a yellow top make more sense (calculated?) than a red top for running a fridge?

 

[xcmountain80]

Well then the only thing that would make the red better was the 3 year coverage/ pro ratingit comes with. That's how all of mine get replaced, pretty much the only reason I have them.